The present invention relates to an image transfer and sheet separation apparatus for use in an electrophotographic system or the like to permit a toner image to be transferred onto a sheet of paper or like medium to a significant efficiency while, at the same time, causing the transfer medium to readily become separated from a photoconductive member, a dielectric member or like latent image supporting member. Further, the present invention is concerned with an image transfer and sheet separation apparatus for use in a two-color electrophotographic copying system or the like which develops latent images of positive and negative polarities with positively charged and negatively charged two kinds of toners, transfers the toner images to a transfer medium and separates the transfer medium from a latent image supporting element.
Ever increasing attention has been paid to a two-color electrophotographic process in which electrostatic latent images charged to the opposite polarities are developed by toners of two different colors. In this type of process, a positive latent image is developed by a negatively charged red toner and a negative latent image by a positively charged black toner, whereafter the toner images are transferred onto a paper sheet or like transfer medium and, then, the sheet is separated from a dielectric member, a photoconductive member or like latent image supporting element.
Various methods have heretofore been proposed to form the positive and negative latent images in the two-color electrophotography: a method using styluses to deposite positive and negative charges on a dielectric member, a method charging individual photoconductive layers, which are stacked together and have different spectral sensitivities, to the positive and negative polarities and, thereby, forming positive and negative latent images by one exposure to image light, a method using a photoconductive member on which a photoconductive layer and an insulating layer are stacked successively, a method which charges a photoconductive member to V.sub.s volt, for example, a background area potential to about 1/2.multidot.V.sub.s volt by laser or like means, one video data to V.sub.s volt and the other video data to about 0 volt. Although the last-mentioned method does not provide positive and negative latent images in a strict sense, it may be included in the class of positive-negative latent image forming methods taking the background potential as a criterion.
Known methods for simultaneous transfer of positive and negative toner images may generally be classified as: one which precharges them to a common polarity before an image transfer and then transfers them by corona charge, and one which after the precharging, causes a charged insulating member or a conductive member impressed with a voltage to contact a transfer medium from the back. The former method is relatively simple and convenient. However, supposing that the polarity of the precharge is positive, the former method fails to efficiently transfer the toner originally charged to the negative and, particularly, causes local omission of transfer in a relatively wide image area, which is critical to the reproduction of quality images. The latter method, on the other hand, renders the separation of a transfer medium from a photoconductive element unstable though superior to the former concerning the quality of images.
Meanwhile, various methods and apparatuses are known to the art for separating a paper sheet or like transfer medium from a latent image supporting member of the kind described. The sheet separation, however, involves problems which are very difficult to solve. In a method in commercially extensive use today, a paper sheet undergone an image transfer has its leading end subjected to AC corona charge to have the charge thereon neutralized and, then, a pawl or like separator means located adjacent a photoconductive member forcibly removes the sheet which is now easily separable due to the neutralized charge. A drawback inherent in this method is that the AC corona charge disturbs an image adjacent the sheet leading end while the pawl also tends to disturb the image or even to damage the photoconductive member. In another known method, after a transfer by corona charge, a conductor which is grounded is brought into contact with a paper sheet from the back to separate a leading end portion of the sheet, whereafter the conductor is impressed with a voltage. This is neither acceptable because an image in the leading end portion of the sheet would become lower in density or entirely omitted. While a paper sheet may be separated by air suction as has also been proposed, this not only fails to stably separate the sheet but generates noise due to the suction.
Means for efficiently transferring a toner of a single polarity and separating a transfer member stably from a photoconductive member is also available in various forms. However, none of them is satisfactory for all the modes of operation designed for an electrophotographic copying machine. Indeed, there has been proposed in connection with an electrophotographic copying machine or the like, which is operable in various modes, to selectively activate individual units or elements of the machine or to change the voltages or polarities depending on the operation mode. This, however, is reflected by the intricacy of construction of the entire apparatus, higher production cost, lower reliability and other various drawbacks.